This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Digital watermarks have been proposed and used for copyright protection of signals such as audio, video, and images. An objective of such watermarking systems is to hide an auxiliary signal within a host signal in such a way that it is substantially imperceptible, and at the same time, difficult to remove without damaging the host signal. The auxiliary signal may carry information that is used to carry out copyright protection mechanisms to varying degrees. For example, the auxiliary signal may merely comprise a “no copy allowed” indication that, once detected and interpreted by a compliant copying device, prevents copying of the host signal. Additionally, or alternatively, the embedded auxiliary signal may carry information that identifies one or more of the rightful owner, author, title or serial number of the host signal. The information contained in the auxiliary signal can also be used for other applications, such as to monitor the usage of embedded content, resolve ownership disputes, keep track of royalties, and the like.
Another application related to digital watermarking involves distinguishing different copies of the same host signal by embedding a unique watermark value into individual copies of a host signal. These applications are sometimes referred to as forensic marking (because the unique watermarks may be used to trace the content to an offending party), transaction marking (because the unique watermarks can identify each legitimate transaction), or fingerprinting (because the unique watermarks can identify perceptually similar host signals, much like fingerprints can identify individuals). Once a content is forensically marked, the embedded watermarks may be used to identify the original source (i.e., legitimate purchaser) of a content, and monitor the subsequent spread of that content through piracy channels. For example, a pirate may purchase a music track over the Internet from a legitimate distributor, directly or using a proxy. Then, the pirate may resell or otherwise redistribute the content in an unauthorized fashion. A similar scenario can occur in the distribution of video or other types of content such as still photos, computer graphics, computer games, and the like that may be distributed over the Internet, or in case of video or music, over “pay-per-view” channels in a cable or satellite TV network. Similarly, copyrighted content that is distributed internally within a content production/distribution entity, or to reviewers or critics prior to their public release, may be illegally used or redistributed. In all of the above cases, it is important to identify the offending party and recover and/or prevent further unauthorized dissemination of the content. Often the fact that forensic marks are present in the content is enough to deter such illegal activities.
Efficient design of a forensic marking procedure is often an important consideration of forensic marking systems since the same host signal often needs to be marked many times. On the other hand, efficient detection of embedded marks may not be a critical consideration for forensic marking systems since watermark detection often takes place only when an illicit activity is detected or suspected. This feature of forensic marking systems may be contrasted to copy-control watermarking systems, where the emphasis is typically on providing a simple extraction method that is easily implemented and carried out in consumer devices, while the embedding (or marking) procedure can be more elaborate and computationally expensive.
Efficiency of a forensic marking system is further complicated due to the fact that a host content (e.g., music, image or video), especially a content that is distributed over the Internet, is often stored and distributed in a compressed format (e.g., MP3 format for music files). In a conventional marking system, the forensic marks are typically applied by first decompressing the host content, embedding the appropriate marks, and re-compressing the content prior to distribution or storage. This procedure has several disadvantages. First, since most compression algorithms involve lossy operations, each round of content decompression and re-compression may further degrades the perceptual quality of the host content. Second, decompression, and, particularly, compression operations are computationally expensive. Therefore, it may not be computationally feasible to decompress and re-compress a content in response to each individual purchase request of that content. It is thus advantageous to apply forensic marks without decompressing and re-compressing the host content.